1. Field of the Invention
This invention relates to a range finder that can be adapted to an optical device such as an auto-focus camera.
2. Description of the Prior Art
The principle of range or distance determination in a conventional range finder is illustrated in FIG. 1 of the accompanying drawings, in which numeral 1 designates object, 2 and 3 lenses, 4 focal plane of the lenses, 5, 6 and 7 images, 8 a first train of light receptor elements, and 9 a second train of light receptor elements.
The object 1 forms its images 5 and 6 on the focal plane 4 through the respective lenses 2 and 3. In case the object 1 is at infinity, the light ray therefrom, which is incident on the lens 3, passes along the optical pass l to form an image 7 on the focal plane 4. Therefore, if the interval x between the two images 6 and 7 can be detected, the distance a to the object 1 can be determined from the following equation, by making use of the known triangulation techniques: ##EQU1## wherein f.sub.e is the focal length of the lenses 2 and 3, and B is the distance between the optical axes of the respective lenses 2 and 3. For obtaining a clear image on the focal plane, the focal length f.sub.e is set so as to satisfy the relation of f.sub.e &lt;&lt;a. Usually, the object image 7 at an infinite distance is unaccessible, so that in this case the image 5 formed by the lens 2 is used. For determining the positions of or the interval between these images, a first and a second train of light receptor elements 8 and 9 are disposed in the neighborhood of the positions where the respective images are formed on the focal plane 4 by the lenses 2 and 3, and in these trains of light receptor elements, those elements which form the same image point with the object being supposed to be at an infinite distance are considered as pairing elements, and said distance determination is made from the correlation or comparison of the outputs of said respective receptor elements based on said pairing elements.
FIG. 2 is a block diagram illustrating the mechanism of a range finder which has been commonly used for determining the interval x between the object image 7 at an infinite distance such as mentioned above and the real image 6. In this diagram, numerals 8 and 9 designate the trains of light receptor elements same as shown in FIG. 1, 10 and 11 a series of binary-coding circuits, 12 and 13 shifts registers, 14 a series of coincidence detection circuits, 15 a counter, and 16 a decision circuit. The analog outputs of the respective light receptor elements in the trains 8 and 9 are discriminated either "0" or "1" on a pertinent threshold level by the binary-coding circuits 10 and 11 and written in the shift registers 12 and 13. 12 and 13 may not necessarily be shift registers, but it is desirable that at least one of them is a shift register. The outputs of the respective bits of these shift registers 12 and 13 are applied to the coincidence detection circuit series 14 in a predetermined combination such as mentioned before. Each circuit in said coincidence detection circuit series 14 generates "1" when the two inputs are same and "0" when they are different. Of the outputs of the coincidence detection circuit series 14, the number of "1" is counted by the counter 15 and given to the decision circuit 16. The decision circuit 16 stores this number, and then it shifts the shift register 12 or 13 and again reads and stores the output of the counter 15. Such shifting of the shift registers 12, 13 and reading/storing of the counter outputs are repeated a predetermined number of times, and the greatest of the counter readings stored is determined. Here, the images on the respective receptor element trains 8 and 9 have the greatest coincidence, and the number of times of shifting of the shift register from the initial state at which said maximum coincidence is given corresponds to x. In such case, however, if the outputs of the receptor elements are processed directly, there is a risk of misoperation due to noise or variations of elements.
FIG. 3 is the diagrams for illustrating the influence of noise on the receptor element output. In the diagrams, the element position is plotted as abscissa and the element output as ordinate.
No serious problem arises in case the noise is superposed at a same rate on both of the output waveform of the receptor element train 8 such as shown in FIG. 3(a) and the output waveform of the receptor element train 9 such as shown in FIG. 3(b), but in case an offset or ramp type noise as shown in FIG. 3(c) or FIG. 3(d) is superposed on the output waveform of one of the receptor element trains, for example, the train 9, the whole or a substantial portion of the output value rises above the threshold as shown in FIG. 3(e) or FIG. 3(f), so that if the two are compared in this state, a misjudgement will surely be committed.
Hitherto, the binary-coding circuit for the outputs of the light receptor elements has been constructed, for instance, as follows.
FIG. 4 is a circuit diagram exemplifying such conventional binary-coding circuit. In the diagram, only a circuit for one bit is shown, but actually such unitary circuits are provided a desired number to form a binary-coding circuit train corresponding to the train of light receptor elements. In the drawing, numeral 17 refers to a photo diode (light receptor element), 18 and 19 switching transistors, 20 a capacitor, and 21 an inverter.
In operation, first the switching transistor 18 is turned on by CLEAR input to effect discharge of the capacitor C. Then, said switching transistor 18 is turned off by CLEAR input while the switching transistor 19 is turned on by input G. Concequently, a current i substantially proportional to the light intensity flows into the capacitor 20 from the photo diode 17 through said switching transistor 19. Upon passage of a certain period of time t after energization of the switching transistor 19, the input G is operated to deenergize the switching transistor 19. At this point, the capacitor 20 is loaded with an electric charge of about i.times.t, and as a consequence, a voltage of Vin=it/C is applied to the input of the inverter 21. Supposing the threshold voltage of the inverter 21 is Vth, the inverter output value is "0" if Vin.gtoreq.Vth and "1" if Vin&lt;Vth. Such change of output value according to the relation between Vin and Vth can be reversed by connecting another inverter next to said inverter 21. What is important here is the conduction time t of the switching transistor 19. If such conduction time is too long, the capacitor 20 is overchanged and the inverter input exceeds the threshold voltage in all of the light receptor elements, while if said conduction time is too short, the capacitor 20 is little charged and hence the inverter input can not exceed the threshold voltage in any receptor element. Thus, if the binary coding is accomplished without giving consideration to the time t, only a pattern of whole "0" or whole " 1" is provided and no information can be obtained. Therefore, the optimal control of the time t must be made by giving consideration to the amount of light received by the whole train of light receptor elements, but this type of control is generally complicated and troublesome.